There Goes the Sun

There Goes the Sun

On the verge of solar maximum, the August 11, 1999
solar eclipse promises to dazzle millions in the path of totality.

"There is nothing in the whole of Nature to rival
the glory of a total eclipse of the Sun." Michael
Maunder & Patrick Moore, 1998, The Sun in Eclipse

August
5, 1999: On August 11, 1999, millions
of people in the Northern Hemisphere will have one last chance
in this millennium to marvel at a total solar eclipse. The 50
mile wide path of totality will extend from the western Atlantic
ocean, through Europe and Turkey, all the way across India into
the Indian Ocean. At the instant of totality viewers situated
along this narrow path can glimpse some of Nature's rarest wonders
-- "Bailey's Beads," the "Diamond Ring,"
mysterious shadow bands, and, of course, the Sun's ephemeral
corona.

Above: One frame from a 159
KB Quicktime Animation of the Moon's shadow sweeping across
Earth during a total solar eclipse. Credit: Digital
Radiance. Note that this animation is intended to illustrate
the basic features of Solar eclipses and is not to scale.

Solar eclipses happen when the new Moon passes in front of the
Sun. They don't take place every month because of the tilt of
the Moon's orbit. The Moon's orbit around the earth is slightly
elliptical, as is the earth's orbit around the Sun. Therefore,
the Moon and the Sun do not always appear to be precisely the
same size in the sky. The Moon's diameter ranges from 29.3 to
33.5 arcminutes while the Sun's diameter may be anywhere between
31.5 to 32.5 arcminutes. About once a year (on average) when
the Moon passes directly in front of the Sun, and
the Moon is the same size or larger than the Sun, we have a total eclipse somewhere on Earth.

Total eclipses are widely regarded as one of Nature's
greatest spectacles. The chill and sweep of the Moon's shadow,
the sparkle of Bailey's beads, and the sight of the corona blazing
across the darkened daytime sky are life-altering experiences
for some observers. Many become globe-hopping "eclipse addicts,"
die-hard lovers of totality who spend years and thousands of
dollars traveling from one total eclipse to another.

This year's total eclipse on August 11, 1999 could be even better
than usual, thanks to the solar maximum. Every 11 years the sun
undergoes a period of heightened activity. There are frequent
solar flares, lots of sunspots, and the corona expands to many
times its average size. The August 11 solar eclipse will take
place just 9 months before the predicted maximum in mid-2000.
Even now the Sun is very active, raising expectations for an
unusually beautiful eclipse.

Seeing the Sights

Most eclipse watchers consider the corona to be the most beautiful
of all the phenomena of totality. The super-hot outer atmosphere
of the sun glows with the intensity of the full moon when the
sun's disk is completely occulted. Pearly white coronal streamers
sometimes stretch several degrees across the sky during eclipses
near a solar maximum.

Left:
The August 11, 1999 solar eclipse will be visible throughout
much of Europe, Pakistan, India, northeast North America and
parts of Asia in the Northern Hemisphere. The path of totality
is indicated by the yellow and blue line. Credit: F.
Espenak, GSFC/ NASA

Second only to the corona for dramatic impact, by some accounts,
is the Moon's shadow. As totality approaches the shadow can be
seen sweeping across the landscape at over 1000 miles per hour.
According to Michael Maunder and Patrick Moore, in their book
The Sun in Eclipse, "It...gives the impression of
a vast dark cloak rushing toward you and then enveloping you:
the general effect can only be described as eerie." Sometimes,
as the Moon's shadow approaches, "shadow bands" can
be seen. These are poorly understood, wavy lines of alternating
shadow and light somewhat like the ripples on the bottom of a
swimming pool.

The swift arrival of the Moon's shadow transforms the landscape.
The darkness of totality resembles nighttime, and plants and
animals react accordingly. Birds stop singing and may go to roost.
Daytime flower blossoms begin to close as if for the night. The
temperature drops in the coolness of the Moon's shadow. All of
Nature seems to pause for this brief moment of daytime darkness.

There is a special quality to the darkness of a total eclipse.
The sky near the horizon still appears bright, and this distant
scattered light produces a slight reddish glow and unusual shadow
effects. Because the direct light of the Sun is blocked, some
of the brighter stars and planets become visible. Because this
year's eclipse occurs just one day before the peak of the Perseid
meteor shower, sky watchers may also see some rare daytime meteors.

The duration
of totality varies depending on the eclipse and the viewing site.
On August 11, 1999, the maximum duration will be 2 minutes 22
seconds for observers in south-central Romania. One of the longest
eclipses on record will take place in 2009
when the "moment" of totality will endure for over
6 minutes as seen from a spot in the Pacific Ocean.

Right: Just before the Moon completely
covers the Sun tiny specks of light called "Bailey's Beads"
appear. Caused by sunlight shining through valleys on the edge
of the Moon, these points of light are spaced irregularly around
the disappearing edge of the Sun, forming the appearance of a
string of beads around the dark disk of the Moon. Bailey's beads
make their brief appearance up to 15 seconds before totality.
When a single point of sunlight remains, a beautiful "diamond
ring" effect is created against the outline of the Moon.
This final sparkling instant signals the arrival of the moon's
shadow. Bailey's Beads and the Diamond ring are seen again in
reverse order at the end of totality when the Moon moves away
from the Sun. During totality colorful prominences and the reddish
chromosphere are also frequently visible. The animation pictured
here is based on art by Duane Hilton.

It's beautiful, but is it good for anything?

Educators!

Please visitThursday's Classroom
for lesson plans and activities related to the August 11, 1999 total
solar eclipse.

Scientists have used solar eclipses as an important
research tool for hundreds of years. Eighteenth century astronomers,
including Edmund Halley, routinely monitored eclipses to refine
the orbits of the Moon and Earth. In the 19th century, astrophysicists
realized that the chromosphere, prominences, and the corona were
important physical aspects of the sun, and they followed eclipses
as the only way to study these intriguing phenomena. In the early
20th century, solar eclipses provided crucial tests of Einstein's
then-new successful theory of General Relativity.

Today, with space telescopes and modern coronagraphs it is sometimes
thought that eclipses have little to offer in the way of science.
This is not so.

To limit the scattering of sunlight, space-borne coronagraphs
have had to block out the inner corona, and ground-based observations
of the corona (excluding eclipses) cannot see the corona very
far from the Sun. The Moon is still Nature's greatest coronagraph!

Eclipses are also unbeatable ways to precisely measure the Sun's
diameter. Members of the IOTA routinely travel to solar eclipses
which they observe near the edge of the path of totality. By
stretching a team of observers perpendicular to the expected
edge of the shadow's path they are able to measure the location
of the edge with a precision to less than 100 meters. This translates
into a measurement of the Sun's diameter with a precision of
only 0.004 arcseconds, or 20 miles. According to such measurements,
the Sun may have been 0.4 arcseconds larger in 1983 than it was
in 1979. Is the Sun oscillating? It's too soon to say. Some of
the eclipse measurements are controversial, and more data are
needed. IOTA members will be on hand for the August 11, 1999
eclipse and they intend to continue their work during future
eclipses as well.

Some
of the most fascinating eclipse research has nothing to do with
astronomy. Biologists and zoologists sometimes use eclipses to
study the circadian rhythms of living creatures. Between 1954
and 1975, two Polish zoologists, R. Wojtusiak and Z. Majlert,
conducted a unique set of experiments in which they observed
the behavior of mammals, birds, and insects during seven eclipses
with varying degrees of coverage, including totality. They discovered
that the daily habits of mammals were little affected by the
eclipse, but that birds and especially insects were influenced.
Under the Moon's shadow, many species of birds manifested anxiety
and roosting behavior, and nearly all birds stopped singing (which
contributes to the oft-noted quietude of totality). Insects are
affected even more. During eclipses studied by the Polish team,
bees returned to their hives en mass, nocturnal moths appeared,
and butterflies settled in the grass as though it were night.
The most sensitive species, bees, have been observed to return
to their hives during partial eclipses covering only 19% of the
Sun!

Other biologists have followed the ground-breaking experiments
of the Polish team with observations that confirm the influence
of eclipses on the behavior of birds, insects, and even nematodes
and plankton! More information about these studies can be found
in J.B. Zirker's book, Total Eclipses of the Sun.

There are many other research applications of solar eclipses,
including global gravity measurements, investigations of ionization
and radio propagation in Earth's atmosphere and studies of asteroid
dust and cometary debris in the innermost solar system. The full
range of research opportunities presented by a solar eclipse
is too broad to review here. Suffice it to say that solar eclipses
promise great value to scientific research for many, many years
to come.

Eclipses on other planets

If the Moon were slightly smaller than the Sun, the best eclipses
would be annular ones with no dramatic corona or blackening of
the daytime sky. If the Moon were slightly larger, the full glory
of the chromosphere and prominences surrounded by the glowing
corona would be lost. Fortunately, the Moon is "just right."

Earth is the only planet in the solar system with spectacular
solar eclipses. Thanks to an apparently improbable coincidence,
the Sun and the Moon are almost exactly the same size as seen
from Earth. The Sun is 400 times larger than the Moon, but it
is also 400 times farther away. The table below, adapted from
The Sun in Eclipse by Maunder and Moore, shows that there
is no other planet where the angular diameter of a satellite
is so close to that of the Sun. The most interesting case may
be Jupiter's moon Amalthea, which subtends an angle of 7' 24"
as seen from the cloud tops of Jupiter, and where the sun is
nearly the same size at 6' 09". Still the sizes of Amalthea
and the Sun are quite different compared to the similarity between
the angular sizes of the Sun and our own satellite. Another interesting
case, noted by Maunder and Moore, is that of Mars's satellite
Phobos, which subtends an angle up to 12.3 arcminutes as seen
from the surface of the red planet. Phobos transits the Sun about
1300 times in every Martian year and each time it does there
would be a hole in the middle of the 21' Sun near "totality."
Maunder & Moore argue that the novelty of such a sight would
soon wear off for future Martian colonists because it happens
so frequently.

As seen from

Object

Apparent diameter

Mars

Sun

21'

Phobos

approx. 12'

Deimos

approx. 2'

Jupiter

Sun

6' 9"

Amalthea

7' 24"

Io

35' 40"

Europa

17' 30"

Ganymede

13' 06"

Callisto

9' 30"

Saturn

Sun

3' 22"

Mimas

10' 54"

Enceladas

10' 36"

Tethys

17' 36"

Dione

12' 24"

Rhea

10' 42"

Titan

17' 10"

Uranus

Sun

1' 41"

Miranda

17' 54"

Ariel

20' 54"

Umbriel

14' 12"

Titania

15' 00"

Oberon

9' 48"

Neptune

Sun

1' 04"

Triton

26' 13"

Based on data from Chapter 14 (pp 183-184) of
The Sun in Eclipse by Michael Maunder and Patrick Moore.

It has often been asked if the similarity of the Moon's and
Sun's diameters can be simple coincidence. In the absence of
more data about the statistical distribution of sizes of stars,
planets, and moons in solar systems other than our own, it would
seem that the most likely answer is "yes." Nevertheless,
it is a fortunate coincidence for the denizens of Earth.

For more information about the August 11, 1999 solar eclipse,
please see Goddard Space Flight Center's Solar
Eclipse home page.